Quite right, they are setting up the standing rigging, in this case the ratlines that run from the side of the ship up to the crosstrees. These act as lateral support for the masts and, with the addition of horizontal ropes at intervals, the name of which I have forgotten, act as a ladder to ascend the mast. Rattling down is the process of adjusting them for tension, very important as too tight will "wring" the mast as the ship moves, too loose and they don't serve the purpose. Rattling is derived from ratlines.
And see http://www.hnsa.org/doc/luce/part3.htm
for a superbly detailed description of rigging a sailing ship. The principles are the same.